offer function-level control over tracing (#5194)

* wiggle: fix compilation with async functions when tracing is off

Fixes #5202

* switch tracing config from a boolean to a struct

This will enable more complex tracing rules in the future

* rename AsyncConfField to FunctionField

It is going to be reused for cases other than just async functions

* add support for disabling tracing per-function

This adds a `disable_for` syntax after the `tracing` boolean.  For
example:

```
wiggle::from_witx!(
    tracing: true disable_for {
        module1::foo,
        module2::{bar, baz},
    }
)
```

crates/wiggle/macro/src/lib.rs

@@ -153,7 +153,7 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
         &config.async_,
         &doc,
         config.wasmtime,
-        config.tracing,
+        &config.tracing,
     )
     .expect("validating codegen settings");
 
@@ -195,7 +195,7 @@ pub fn wasmtime_integration(args: TokenStream) -> TokenStream {
         &config.c.async_,
         &doc,
         true,
-        config.c.tracing,
+        &config.c.tracing,
     )
     .expect("validating codegen settings");